1. Field of the Invention
The present invention relates to the manufacture of semiconductor devices and particularly to four region semiconductor switches such as thyristors.
2. Description of the Prior Art
It is known that the switching speed of semiconductor switching devices, such as thyristors, generally depends upon carrier lifetime.
Methods of shortening the lifetime of carriers in semiconductors which have been employed in the prior art include doping with a heavy metal or irradiating with radioactive radiation.
Copper, iron and gold are examples of heavy metals known to reduce carrier lifetime in semiconductors. For example, see U.S. Pat. No. 3,556,879. Gold, however, is the preferred dopent for use with silicon semiconductor devices.
More specifically, to shorten switching turn-off time, recombination centers are formed in the PNPN silicon element by diffusing gold into the silicon in its entirety or by irradiating the silicon with radioactive radiation of sufficient energy to cause recombination centers to occur in the silicon crystal lattice.
Although irradiation is preferred to gold-doping in the respect that irradiation does not produce the collateral effects on characteristics such as blocking leakage current, the irradiation method suffers from the disadvantage that temperatures in the range of 200.degree. C. to 300.degree. C. occurring during operation of the device will eventually anneal the silicon to the extent that the irradiation produced recombination centers gradually vanish. Temperatures of such magnitude (200.degree. C. to 300.degree. C.) are produced in the initial conduction portion of the device in the vicinity of the gate due to a concentration of current which occurs during the switch's turn-on mode.
Accordingly, the carrier lifetime in irradiated switches increases in duration with the continued operation of the device. That is, as the number of switchings increases, the turn-on time of successive switchings increases. On the other hand, the gold-doping method of producing recombination centers in a switching device also, as noted above, increases the leakage current at high operating temperatures as a result of the effects of doping on the impurity level in the vicinity of the PN junction which blocks applied voltage. This increase in leakage current is proportional to the gold impurity level or concentration in the vicinity of the blocking junction.
Methods of selective irradiation of semiconductor devices are known in the prior art, examples of such methods being set forth in U.S. Pat. Nos. 3,840,887 and 3,852,612. Methods of selective gold doping of semiconductor devices are also known in the art, an example of which is U.S. Pat. No. 3,440,113. The present invention combines the advantages of selective gold doping with the advantages of irradiation to produce a semiconductor switching device with superior electrical characteristics. In a present preferred embodiment of the invention a four region switching device or thyristor device is disclosed which exhibits stability with respect to the fast switching characteristics without compromising other characteristics.